Thermoplastic molding compositions are based on a thermoplastic resin and are widely used for the injection molding or extrusion of many products. In many applications it is necessary to add materials to the composition which will impart flame retardance to the molded article. In the prior art many additives have been used for this purpose. For example, U.S. Pat. No. 3,915,926 discloses a composition of a polycarbonate and a brominated flame retardant in combination with an antimony compound. Generally, it has been found that when a brominated compound has been used as the sole flame retardant additive, the composition has not been rendered sufficiently flame retardant so that it passes the Underwriters Laboratory No. 94 test and receives a V0 rating. The antimony compound was added to the composition to act as synergist for the brominated flame retardant to obtain a V0 rating.
Organic phosphates have also been added to thermoplastic compositions to impart flame retardant properties. It is theorized that the organic phosphates act as flame retardants because they volatilize when exposed to the intense heat that is generated by contact with an open flame and form a protective barrier close to the surface of the thermoplastic material in which they are dispersed.
Copending application Ser. No. 07/994,794, filed Dec. 22, 1992 (now abandoned), by Richard C. Crosby and Douglas G. Hamilton discloses that polyacid pyrophosphates are used to stabilize polyester-polycarbonate compositions.
U.S. Pat. No. 5,367,011, filed by Eileen Walsh discloses that polyacid pyrophosphates are used to stabilize homopolymer blends of polyester compositions.
It has been surprisingly discovered that thermoplastic compositions may be rendered flame retardant, obtaining a V0 flame rating under Underwriters Laboratories Test No. 94, by the addition of effective amounts of a metal acid pyrophosphate and a halogenated flame retardant. This is unexpected because metal acid pyrophosphate compounds are inorganic and are not considered volatile. It has also been found that this flame retardance is obtainable without the use of an antimony oxide synergist.